Sinkholes are rare in California, but the heavy rains this winter produced several dangerous ones around the state.

A sinkhole is an area of ground that has no natural external surface drainage – when it rains, all the water stays inside the sinkhole and typically drains into the subsurface. There are several types of sinkholes that can vary from a few feet wide to hundreds of acres and from less than 1 to more than 100 feet deep.

Areas prone to natural sinkholes

The above USGS map shows areas of the United States that have certain rock types that are susceptible to dissolving in water. In these areas underground cavities can form, creating catastrophic sinkholes.

These rock types are evaporites (salt, gypsum and anhydrite) and carbonates (limestone and dolomite).

Sinkholes can be human-induced as well. Examples include decaying infrastructure, collapsed mines and abandoned septic tanks.

One type of sinkhole is a cover-collapse sinkhole which can develop abruptly (over a period of hours) and cause catastrophic damage. They occur where the covering sediments contain a significant amount of clay.

Warning signs

In buildings, look for:

Doors and windows that don’t close properly, which can be the result of movement of the building’s foundation.

Structural cracks in walls and floors.

Muddy or cloudy well water.

On the property, check for:

Formation of small ponds as rainfall accumulates in new areas.

Slumping or falling trees or fence posts.

Previously buried items such as foundations, fence posts and trees becoming exposed as the ground sinks.

Gullies and areas of bare soil, which are formed as soil is carried towards the sinkhole.

A circular pattern of ground cracks around the sinking area. Sudden earth cracking should be interpreted as a very serious risk of sinkhole or earth collapse.

Localized, gradual ground settlement.

Interrupted plumbing or electrical service to a building or neighborhood due to damaged utility lines.

Vegetation that wilts and dies as essential water is drawn away by the sinkhole.

Are you covered?

Most homeowners insurance policies do not cover sinkholes in California. You can buy separate policies for earthquakes, flooding and landslides.

In Florida, sinkholes are quite common and insurance options include coverage for sinkholes and “catastrophic ground cover collapse.”

Potholes vs. sinkholes

In Southern California, potholes often result from a combination of rain and traffic.

Sources: The USGS Water Science School, International Association of Certified Home Inspectors, Insurance SCV

Kurt Snibbe is a visual journalist for Southern California News Group. Snibbe has won several medals in international graphics competitions and was a staff cartoonist for ESPN.com. Snibbe began with The Orange County Register in 1997, left in 2008 and returned in 2013.